1. The Progress and Meaning of Apo2L/TRAIL for Tumor Therapy
Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the superfamily of tumor necrosis factors (TNFs), the gene sequence of which was obtained through independent cloning by Wiley et al. in 1995 and Pitti et al. in 1996; the latter named TRAIL as Apo2 Ligand (Apo2L). Studies later showed that Apo2L and TRAIL are the same kind of protein in nature, therefore, it customarily can be called as Apo2L/TRAIL. TRAIL is firstly used as a conditioning agent for the congenital or acquired immunity of living organisms; then, it plays its role of anti-tumor as the immunological surveillance in the exogenous apoptotic pathway. The greatest benefit of TRAIL is that it can induce apoptosis for various tumors selectively while having hardly any toxicity to normal cells. It is shown from research data that no matter in vitro or in vivo, Apo2L/TRAIL has apoptosis-inducing effect on the human cancer cell lines from various sources, including colon (rectal) cancer, lung cancer, breast cancer, prostatic cancer, pancreatic cancer, renal cancer, CNS tumor, thyroid cancer, lymphoma, leukemia and multiple myeloma, etc.
TRAIL has always been developed as an important potential antineoplastic drug in almost 20 years since it was discovered, the clinical trial of which has entered Phase II in foreign countries and has been finished Phase III in China. A large number of in vitro and in vivo trials verified that TRAIL has tumor specific cytotoxicity and has obvious synergistic effect especially when it is used with small dose of chemotherapy drugs. On the contrary, the studies found that the TRAIL tolerance caused by lack of apoptosis mechanism in living organisms has definite relationship with the rapid growth and transfer of tumor cell.
Tumor is a group of disease with high heterogeneity. The traditional genotyping methods as per tissues and organs and pathologic changes are no longer suitable for tumor diagnosis and treatment. The current research direction is aiming at elucidating genetic expression and molecular subtyping of different tumor cells, so as to provide more targeted treatment to patients. It is realized with better understanding of antineoplastic drugs that, the functioning process of no matter cytotoxic drug, molecular targeted drug or monoclonal antibody involves the activation of tumor cell apoptosis pathway; the signal pathway for inducing apoptosis of tumor cells is the hub and the key link for these drugs to function, and the apoptosis evasion is the critical mechanism for tumor development and drug resistance.
2. Defects and Countermeasures of Apo2L/TRAIL for Tumor Therapy
The recent development shows that it is far from enough to cure various types of tumor with Apo2L/TRAIL only. Although the agonistic monoclonal antibody of recombinant human Apo2L/TRAIL or TRAIL receptor DR4/DR5 has achieved encouraging results in clinic treatment of Phase I, no definite clinical benefit is showed in the subsequent clinical study of Phase II. A large number of studies show that normal cells and almost more than a half (even 60%) of passage tumor cell strains have shown drug resistance to TRAIL. According to Roberta di peitro and Giorgia zaulim, the antibiotic sensitive rate of Apo2L/TRAIL to 61 strains out of the studied 92 strains of primary or passage tumor cells is 66.3% and the rest 31 strains 33.7%. The tolerance of TRAIL to normal cells has certain physiological significance. Maintaining precise regulation and control effect in vivo, TRAIL only removes the aging, degenerated and transformed cells in the process of growth and development while not killing any normal cells. Almost all TRAIL sensitive tumor cells have similar soundness and functions in each link and factor in the apoptotic signal pathway, while each TRAIL drug-resistant tumor cell has defects and variations in some links and factors in the apoptotic signal pathway. These defects and variations making the apoptotic threshold of the drug-resistant tumor cells increased abnormally, thus the tumor cells can easily escape from apoptosis removal and keep on growing and proliferating.
A large number of studies verified that the independent use of Apo2L/TRAIL does not have significant inhibitory and killing effects to many tumor cells. The reason for that is that the apoptotic signal pathway of tumor cells is a huge and complex system, containing both many apoptosis-inducing factors and a lot of apoptosis inhibitors. The interaction between the two factors determines the final destination of tumor cells. The soundness and function of the apoptotic signal pathway are of necessary condition but not sufficient condition for tumor cell apoptosis. Many different types of drugs, molecules or genetic interference may increase the sensibility of TRAIL to tumor cells. These drugs comprise different types of chemotherapy drugs, natural products and small molecule kinase inhibitors, etc. They strengthen the activity of TRAIL-induced tumor cells apoptosis by strengthening the extracellular apoptotic signal pathway (e.g.: up-regulating death receptors (DRs) expression, enhancing the aggregation and redistribution of DRs in upper lipid raft micro domain of cell membrane, enhancing the endocytosis of TRAIL/DRs compound on the cell membrane, promoting recruitment of TRAIL/DRs complex by DISC, activating the activity of Caspase (Caspase 8) at initial period and inhibiting the activities of apoptosis antagonists as FLIP, XIAP and IAPs) or the mitochondria apoptotic signal pathway (e.g.: strengthening the depolarization of mitochondrial membrane potential, promoting increasing mitochondria permeability and releasing Cyt c, Smac or ARTs, promoting Bid splitting into tBid, promoting oligomerization of Bax and Bad and inhibiting the apoptosis antagonists as Bcl-2, Bcl-xL, Bcl-w, Mcl-1 and Mcl-1) or inhibiting other cell survival signal pathways (such as ERK/PI3K/AKt, MEK, Jak-STAT 3, MAPK and NF-κB) or combining several pathways.
Although the development process of TRAIL and its receptor agonistic monoclonal antibody is frustrated temporally, with fully elucidation of the apoptotic signal pathway, and fully disclosure of the conversion relation of apoptosis and tolerance, the development of targeted antineoplastic drugs based on apoptotic signal pathway is never stopped. The current studies are mainly focused on combining the application of TRAIL and cytotoxic agent, however most experiments show that such combination will only have obvious synergistic effect on the tumor cell which is relatively sensitive to TRAIL and cannot reverse the resistance caused by various different resistance mechanisms. TRAIL and cytotoxic agent belong to two types of drugs, having differences in drug type, dosage, administration route and mode of action and little possibility for being developed into a new simplex, stable and controlled drug; meanwhile, after TRAIL and cytotoxic agent are combined, the toxic and side effect will still exist, therefore, it has no distinct advantage.